Distributed control networks, operating over various media, are becoming more prevalent as the cost of these systems comes down, and the capability and reliability of these systems improve. It has become desirable to be able to provide a low-cost, fault-tolerant, robust, compact fiber optic transceiver for such distributed control systems, especially in applications requiring the well-known benefits of fiber optics. Applications for such systems include industrial controls where high electrical noise levels exist, where high voltage differentials exits, or where information must be communicated through hazardous locations. Transportation systems require high noise immunity, small size, and light weight. Building automation systems require lightning immunity when passing between buildings. Security systems require communications paths that do not unintentionally radiate confidential data. Alarm and protection systems require fault tolerance and fault detection capabilities. To allow fiber optics to be used practically in these applications, the apparatus and means disclosed herein were invented.
Receiver power level monitoring, otherwise known as "weak link detection", typically requires diagnostic equipment, diagnostic optics, diagnostic data, and/or diagnostic software in order to monitor operations and/or receive power level. For example, Canadian Pat. No. 2,089,995 discloses a system that monitors signal integrity utilizing infra-red transceiver components, but this system requires local optical feedback (which is expensive for fiber optic transceivers), and it fails to test the integrity of the communications path between a transmitter in one node to the intended receiver in a different node. U.S. Pat. No. 5,220,581 discloses a method whereby the magnitude of the jitter in the received data is captured using non-sequential sorting logic to determine the number of events in each sort counter. Although this method will achieve the desired result, it requires a significant amount of logic to implement the multiple counters and comparators. U.S. Pat. No. 5,396,357 discloses a method whereby the noise output from a discriminator is measured to determine the relative health of the fiber link. Again this requires substantial circuitry to capture and measure the noise in the receiver. Note that "weak link detection" in the context of this description refers to low optical power as measured at a fiber optic receiver, and this should not be confused with weak links in semiconductor and superconductor structures.
Prior art in pulse width distortion compensation disclose less than ideal correction means. For example, U.S. Pat. No. 5,309,475 discloses a mechanism whereby each node inverts the data being repeated, so the pulse width distortion at each node, which is presumed to be consistent, will cancel through inversion. Unfortunately, the magnitude of pulse width distortion is often inconsistent between nodes, since the magnitude of pulse width distortion can be proportional to the amplitude of the receive optical power at each node. Varying cable length between nodes can result in asymmetrical pulse width distortion, which will also reduce the effectiveness of this correction scheme. U.S. Pat. No. 4,881,041 proposes a feedback correction mechanism that depends on averaging "the direct current component" of the receive pulse widths, and adjusting the receive pulse width until the average pulse width is returned to the nominal 50% level. This method assumes that the average pulse width is 50%, which is not valid if the network is quiet. The DC component in this case will drift to an unacceptably low level, so that when data begins to be received again the pulse width will be unacceptably distorted until the integrator recovers. U.S. Pat. No. 4,675,545 discloses a method where a leading edge detector initiates a sequence where the receive data is sampled one-quarter bit time after the leading edge is detected, and then sampled in one-half bit intervals. The correction range of this method allows for pulse width distortion that is up to .+-.50% of nominal. Unfortunately this method has two major drawbacks. The first is that it introduces a 1/4 bit delay in the repeating of data, regardless of the magnitude of the pulse width distortion. If a large number of nodes are present on a network, then this can significantly add to the propagation delay of a packet around the network. Also, certain fiber optic receivers, including the fiber optic receiver disclosed herein, exhibit pulse width distortion in one direction only, and the magnitude of the pulse width distortion can exceed 50% at high power levels. Therefore the method of U.S. Pat. No. 4,675,545 may result in data errors when receivers are subject to high input optical power.
Although U.S. Pat. No. 5,220,581 recognizes signal jitter as a potential benefit in detecting a weak link, the fact remains that signal jitter when accumulated through nodes in a network may result in data errors.
Since the network can be configured in a loop topology, a means is required to prevent recirculation of packets around a network.
It is an object of the invention to address these or other problems with single fiber transceivers and networks.